Effiective ways to test multi section cap can?

[keithb7]

Hi folks, I did a search here but was unsuccessful. I am wondering how to test a multi section Cap can in a 1968 Princeton Reverb.
Amp seems to play fine, but the cap can is dated 1966. Do these cap cans eventually bubble and blister like normal caps?
What is the proper way to tell if they are due for replacement, other than just a date code? I have a DMM that can measure
capacitance, need to brush up on that. Maybe that'll tell me? I am aware of the stored charges and have experience safely discharging
caps. Thanks in advance.

I am learning here so bear with me please. Any help appreciated.
As a trial I have some old removed 25uf/25v Mallory white caps from 1968. Single caps. I attempted to measure
them with my DMM. I have measured several and come up with 34 uF . Do values rise as the caps wear out?
As shown in this example.

Thanks, Keith

[nashvillebill]

Listen for the sound the cap makes when it lands in the bottom of a trash can. If it makes a "thunk", it needs replacing. If you don't hear a sound, get your hearing checked, and replace the cap.

They're approaching 50 years old, well past their "best when used by" date. And yes, I've seen old caps that measured higher capacitance as they dried out.

[nickb]

Hi folks, I did a search here but was unsuccessful. I am wondering how to test a multi section Cap can in a 1968 Princeton Reverb.
Amp seems to play fine, but the cap can is dated 1966. Do these cap cans eventually bubble and blister like normal caps?
What is the proper way to tell if they are due for replacement, other than just a date code? I have a DMM that can measure
capacitance, need to brush up on that. Maybe that'll tell me? I am aware of the stored charges and have experience safely discharging
caps. Thanks in advance.

I am learning here so bear with me please. Any help appreciated.
As a trial I have some old removed 25uf/25v Mallory white caps from 1968. Single caps. I attempted to measure
them with my DMM. I have measured several and come up with 34 uF . Do values rise as the caps wear out?
As shown in this example.

Thanks, Keith

I think that the best guide to the condition of an electrolytic cap is the equivalent series resistance (ESR). The older caps tended to dry out. You can buy meters for this and there are mixed opinions on whether it's worthwhile. I do have one and I personally believe it's a great way to spot a cap that is on it's way out and is most effective when you have a bunch of identical caps to compare. For the occasional fixer, it's probably better to simply replace when they are that old and era you as usually want reliability first of all. That said, the manufacturing techniques got better somewhere around the late 1960's, I can't remember where I heard that, and I've seen plenty that old that are still going strong.

IIRC, I've seen the capacitance tend to go down as they fail. Also they can run warm and you can sometimes see a bubble over the vent.

For smoothing caps you can measure the AC volts across them when in circuit to get an idea of how well they are working but the expected reading depends on the circuit to some extent and so I'm hesitant to come up with generalized numbers. Of course, excessive hum from the speaker is often an obvious indication of a failing cap.

[Leo_Gnardo]

And yes, I've seen old caps that measured higher capacitance as they dried out.

Seen those too, caps that are partially conductive make my cap meter read well over spec, when I see that I get suspicious.

Another angle on the thunk test, if you don't hear the thunk, then empty out your trash pail & try it again.

[Justin Thomas]

If the amp is otherwise in pretty good shape as far as "not much replaced," AND I wasn't gigging with it, AND there is no hum/noise/other, I'd leave it in. I would leave it and gig it IF I had another spare amp that I REALLY liked the sound of. If it was my only gigging amp, I might carry a spare can...

I'd rather trust a 50-yr old original than a 3-yr-old replacement... and I think they're kind of a PITA to needlessly replace. Like trannies...

That said, I like Nashvillebill's test the best of all!

Justin

Edit: as far as capacitance going up, isn't that GOOD? Maybe that's the trick to getting the thunderous lows of a 4x12 in a PR...

[pdf64]

I am wondering how to test a multi section Cap can in a 1968 Princeton Reverb.
Amp seems to play fine, but the cap can is dated 1966. Do these cap cans eventually bubble and blister like normal caps?
What is the proper way to tell if they are due for replacement, other than just a date code? I have a DMM that can measure
capacitance, need to brush up on that. Maybe that'll tell me? I am aware of the stored charges and have experience safely discharging
caps. Thanks in advance.

I am learning here so bear with me please. Any help appreciated.
As a trial I have some old removed 25uf/25v Mallory white caps from 1968. Single caps. I attempted to measure
them with my DMM. I have measured several and come up with 34 uF . Do values rise as the caps wear out?
As shown in this example.

Thanks, Keith

There's often a sealed vent point in the rubber end cap (where the terminals are fitted), noticeable as a slight dimple.
If there's a problem, that may be seem to bulge out or rupture and emit goop.
If it looks fine, stays cool, and does its reservoir / decoupling jobs, consider leaving it be.
Maybe tack a 'known good' small 4.7 or 10uF cap across each section in turn and note ripple voltage before and after each time.
If there's no appreciable improvement (over what may be expected from the slightly increased capacitance value) then call it good.
If it had been standing unused for > 6 months, remove the signal tubes and add a big (eg 10k 10W) resistor between rectifier and reservoir.
Checking the voltage drop across each resistor and calculate the leakage current for each section.
If after a while (may be many hours) they all fall to a low level (eg <2mA dc) then consider it successfully re-formed.

Whereas I'd just replace the any old white / cardboard Mallorys or the black Whales types.
ECaps were / are made to a wide tolerance, eg +50-20%; they may have been 34uF from the factory.
If you really want to check them, tack a good one across it and check for any change in performance, eg if the mid band stage gain increases much when it's cathode bypass is itself bypassed, then the original is likely faulty, eg high ESR.
I've found ones with high ESR can measure significantly over their nominal value on a DMM cap range.

[J M Fahey]

An electrolytic can go up in capacitance and that's a sign of DOOM .

Different to all other caps, where you have 2 metallic sheets separated by a "mechanical" insulator, be it oiled paper, some plastic or even air , (even audiophool caps made out of gold/silver/platinum separated by embroidered silk and sweet smelling chosen beeswax follow that rule), in electrolytics one metal foil is actual aluminum, the other sheet is far away but connected by an electrically conductive liquid (the "electrolyte" ) and the insulator is a layer of complex aluminum oxide chemically grown on the first aluminum foil.

Oxide thickness defines capacitance and voltage resistance/rating.

That chemical insulating layer tends to dissolve in the chemically very aggressive electrolyte, but applied voltage does the proper electrochemical reaction to regenerate it, so caps in constant use tend to hold parameters, both capacitance and voltage rating.

Now leave that amp in storage unused for years, oxide layer parcially dissolves, loses thickness, capacitance increases ... but voltage rating goes down, big way

That's why "reforming" is needed on old unused caps, so the oxide layer thickness rebuilds electrochemically.

So in a nutshell, reading higher capacitance on an old cap is not the good news it seems at first sight.

[Justin Thomas]

Hey Juan,

Don't miss the wink! I was poking fun at all the guys who want huge bottom end out of their tiny little amps; "stiffening up the power supply" with huge caps is always the first suggestion... Just go out and buy the da#n Marshall! I toss those caps if they read insane uF values. But I uusually don't check unless the amp goes "hummmmmmm..."

Thanks for the FACTUAL explanation! Most (nearly all) the cans I see (95% in Fenders) have been in steady use over the years. The voltages and noise levels and uF readings are well within spec, so I leave em in. That stripped organ chassis, on the other hand... I just wish they weren't so dang exensive, cuz I like the looks, and they mount up so easy, no clamps or other junk. Just place, twist, solder. Simple.

Justin

[Enzo]

Also, measuring a cap with your meter is no test. These caps are working at hundreds of volts, your meter works on one or two volts.

[Mick Bailey]

I use my ESR meter pretty much every day and wouldn't be without it for spotting caps that are on their way out when visual signs are OK. Like a ring-tester, it's one of those things that I've grown used to using and is especially useful because mine tests a cap in-circuit and at working voltage.

Old caps can be high in value because of tolerance. It wasn't uncommon even as late as the 70s to see electrolytic caps with a -20%/+80% tolerance.